package suanfa;

import java.util.*;

public class alienOrder {
    Map<Character, Set<Character>> edges=new HashMap<>();
    Map<Character,Integer> in=new HashMap<>();
    boolean check;
    public String alienOrder(String[] words) {
        //1.初始化入度哈希表+购图
        for(String s:words){
            for(int i=0;i<s.length();i++){
                char ch=s.charAt(i);
                in.put(ch,0);
            }
        }
        int n=words.length;
        for(int i=0;i<n;i++){
            for(int j=i+1;j<n;j++){
                add(words[i],words[j]);
                if(check==true) return "";
            }
        }
        //2.拓扑排序
        Queue<Character> q=new LinkedList<>();
        for(char ch:in.keySet()){
            if(in.get(ch)==0) q.add(ch);
        }
        StringBuffer ret=new StringBuffer();
        while(!q.isEmpty()){
            char t=q.poll();
            ret.append(t);
            if(!edges.containsKey(t)) continue;
            for(Character ch:edges.get(t)){
                in.put(ch,in.get(ch)-1);
                if(in.get(ch)==0) q.add(ch);
            }
        }
        for(char ch:in.keySet()){
            if(in.get(ch)!=0) return "";
        }
        return ret.toString();
    }
    public void add(String s1,String s2){
        int n=Math.min(s1.length(),s2.length());
        int i=0;
        for(;i<n;i++){
            char c1=s1.charAt(i),c2=s2.charAt(i);
            if(c1!=c2){
                if(!edges.containsKey(c1)){
                    edges.put(c1,new HashSet<>());
                }
                if(!edges.get(c1).contains(c2)){
                    edges.get(c1).add(c2);
                    in.put(c2,in.get(c2)+1);
                }
                break;
            }
        }
        if(i==s2.length() && i<s1.length()) check=true;
    }
}
